gainging weight

Weight gain due to a "bad weekend" is going to be mostly water retention. Much of what people eat on bad days is full of salt, which causes increased water retention. Plus, you're going to have additional remnants of the meal still in various parts of your digestive system. It is extremely difficult to put on any serious weight over a couple days unless you really make a concerted effort to do nothing but eat. At most, you likely gained a pound of real weight. Most of the rest of what you see will come off once your osmotic balance reasserts itself and digestion catches up to your meals.